The present invention relates to image analysis and especially for image representing a rock in order to determine rock geophysical properties at a microscopic level.
The approaches described in this section could be pursued, but are not necessarily approaches that have been previously conceived or pursued. Therefore, unless otherwise indicated herein, the approaches described in this section are not prior art to the claims in this application and are not admitted to be prior art by inclusion in this section. Furthermore, all embodiments are not necessarily intended to solve all or even any of the problems brought forward in this section.
Determining geophysical properties and petro-elastic properties of the subsoil is important for geologists and engineers to create relevant models of petroleum or gas reservoirs.
This determination may be performed by obtaining an image (2D or 3D image, gray scale color image most of the time) of a piece of rocks obtained through core samples. Micro computed tomography methods may be used in order to such images (or MCT image): micro-scan method or FIPS beam method for instance.
A core plug may be considered to be solid comprising at least one matrix (normally a single matrix) with pores embedded in it.
Once this image is obtained, it is possible to “binarize” the image.
This step of gray scale binarization is very important as it determines the quality and the reliability of the rock properties computed from the MCT image.
Binarization is achieved by defining a threshold value that divide “grain-associated” gray scale values from the “pore-associated” gray scale values. Binarization comprises creating a new two colors image (white and black for instance), points of the new image corresponding to points of the initial image below a given threshold being white (and thus associated with pores for instance), points of the new image corresponding to points of the initial image above the given threshold being black (and thus associated with grain for instance).
If the threshold value is too low, the image pore space is under estimated and if the threshold value is too high, the image pore space is over estimated. Consequently, the geophysical properties of the rock are erroneous if the threshold value is not adequate.
Thus the determination of the adequate value of the threshold is highly important and there is a need for a correct determination.
Some practitioner may determine a lab measured porosity of a rock sample and try to obtain the same porosity by modifying the threshold. This method does not provide accurate result as the porosity of a rock sample may change significantly from one part of a sample to another part of the sample. At the very least, this method is very complex to enable if accurate result is needed. In addition, this method lacks physical basis: for example, it does not guarantee the continuity and therefore the uniqueness of the matrix (the grain-part of the rock sample) but only the porosity value of the rock sample.
There is thus a need for a method that provides accurate and adequate determination of the threshold to provide adequate geophysical properties and/or to guarantee continuity in the matrix of the rock sample.